Measurements using a fast critical assembly were performed in conjunction with the Brookhaven Fast Research Reactor Study. Cores made of highly enriched uranium, stainless steel, and aluminum, reflected mostly by stainless steel were used to obtain broad survey information, and, concurrently, to test the preliminary theoretical model. Quantities measured included critical mass, characteristics of movable reflector blocks, prompt neutron lifetime, worth of materials in potential control positions, neutron yield for a given power, and moderator block characteristics; the first three of these have been compared with calculations.